Optical information recording media as typified by DVDs (Digital Versatile/Video Discs) and Blu-ray Discs are known as information recording media (hereinafter, may be referred to as “recording media” or “media”) capable of recording and reproducing large volumes of information. Increase in the recording density of the DVDs and the Blu-ray Discs has been achieved by reducing the spot diameter through shortening of the wavelength of the semiconductor laser used for recording and reproduction and through increase in the numerical apertures (NA) of the objective lens. However, it is considered that, due to the diffraction limit of light, further increase in capacity by reduction in spot diameter is difficult to achieve by hitherto-used methods.
In recent years, an optical recording method using near-field light has been drawing attention as a technique for breaking through the problem of the diffraction limit. When light is incident, for example, on an opening or a particle having a size equal to or smaller than the wavelength of the light, near-field light is generated locally in the immediate vicinity of the opening or the particle. The spot diameter formed by the near-field light does not depend on the wavelength of the incident light, but is determined depending on the size of the opening or the particle on which the light is incident. In early times, a method had been often employed in which light is made incident, for example, on a fiber probe having a sharpened tip, and thus near-field light is generated in minute openings provided in the tip. However, this method has a problem in that the use efficiency of the incident light is low. In recent years, a near-field light generating element using surface plasmon resonance in a metal has been proposed as a device for significantly improving the use efficiency of light (Patent Literature 1, for example). In this device, recording and reproduction are performed by irradiating a minute metal film with light of an appropriate wavelength to induce surface plasmon resonance, and thus by generating near-field light in the vicinity of the metal film. In addition, for example, methods have been proposed in which patterns are formed in advance on a substrate of an information recording medium so that stable recording and reproduction can be performed even when the recording medium has an increased density (Patent Literature 2 and Patent Literature 3, for example).
Furthermore, an information recording medium using nanoparticles for a recording layer has been proposed (Patent Literature 4, for example). The recording layer of this information recording medium is formed of metal particles having a diameter of 100 nm or less and arranged in such a manner as to be enclosed by a phase-change material whose phase state transits to a crystalline state or an amorphous state in response to irradiation with light. In this recording layer, a material exhibiting localized surface plasmon resonance, such as Pt, Ag, Au, Al, or Cu, is used for the metal particles, and a material whose complex dielectric constant changes depending on its phase state, such as Ge—Sb—Te or Ag—In—Sb—Te, is used for the phase-change material. The nanoparticles make it possible to change the complex dielectric constant of the phase-change material by irradiation with light having an intensity equal to or higher than a predetermined level. The degree of localized surface plasmon resonance generated by the nanoparticles changes in accordance with the change of the complex dielectric constant. Such change of the degree of localized surface plasmon resonance allows recording and reproduction of information.
The use of these techniques make it possible to perform recording with reduced-size recording marks. Consequently, further increase in the density and capacity of an optical memory can be achieved.